Selective and rapid extraction of trace amount of gold from complex liquids with silver(I)-organic frameworks

The design of adsorbents for rapid, selective extraction of ultra-trace amounts of gold from complex liquids is desirable from both an environmental and economical point of view. However, the development of such materials remains challenging. Herein, we report the fabrication of two vinylene-linked two-dimensional silver(I)-organic frameworks prepared via Knoevenagel condensation. This material enables selective sensing of gold with a low limit of detection of 60 ppb, as well as selective uptake of ultra-trace gold from complex aqueous mixtures including distilled water with 15 competing metal ions, leaching solution of electronic waste (e-waste), wastewater, and seawater. The present adsorbent delivers a gold adsorption capacity of 954 mg g−1, excellent selectivity and reusability, and can rapidly and selectively extract ultra-trace gold from seawater down to ~20 ppb (94% removal in 10 minutes). In addition, the purity of recovered gold from e-waste reaches 23.8 Karat (99.17% pure).


Stability in various solvents
JNM-100 or JNM-100-AO (10 mg) was added to a 20 mL glass vial containing 10 mL solution including water, dioxane, methanol, chloroform, DMF, DMSO, 10 M NaOH solution and 1 M HCl solution. The mixture was stand at room temperature for 3 days, after then the JNMs powders were filtrated and vacuum dried at 100 °C for 12 h. The obtained powder was weighed and tested by PXRD and the concentration of silver ions in the filtrate was determined by ICP-MS. Considering the application of gold adsorption, the N2 absorption profiles of samples that after immersing in 1 M HCl, and after 5 cycles of absorption of gold, were recorded.

Fluorescence sensing experiments in pure water
The stock solution of JNMs (0.3 mg/mL, 500 µL) prepared by dispersion of JNMs in water was added to the solution which contains different amounts of Au 3+ (200 µL), where the KAuCl4 was used as gold sources, then diluted to 2 mL water in a quartz cuvette. JNMs was readily dispersed in water and the obtained suspension was almost transparent. The fluorescence spectra were recorded immediately after an appropriate aliquot of the stock solution of ions was added. All measurements were excited at λex = 460 nm and the corresponding emission wavelengths were tested from λem = 500 to 800 nm unless otherwise stated.

Recovery of PL intensity of JNMs
The

(Supplementary Equation 4)
Where is the adsorption capacity (mg g -1 ) at a predetermined time t (min) and is the equilibrium adsorption capacity (mg g -1 ). 1 (min -1 ) and 2 (g mg -1 min -1 ) is the rate constant of pseudo-first-order and pseudo-second-order adsorption, respectively.
The distribution coefficient (Kd) value as used for the determination of the affinity and selectivity of sorbents for Au(III) (mL g -1 ), is given by the Supplementary Equation 5:

(Supplementary Equation 5)
Where V is the volume of the treated solution (mL), m is the amount of used adsorbent

NOTE 3!:
The different brand of CPUs will give different gold concentration in leaching solution as shown in Supplementary Table 9. Moreover, we also tried aqua regia system for comparison, which also gave similar results with variation of gold concentration in leaching solution. Combined these results, to obtain the maximum of gold, the usage of NBS/Py should be excessive (at least more than 150 mg/50 µL per one CPU).  Figure 47 is used for comparison, and the volume of leaching solution is 100 mL.

Gold recovery from electronic waste treated with NBS/Py
Although the e-wastes are commonly pretreated using aqua regia as leaching solution in industry, considering the stability of JNMs in aqua regia and environmental issues, the N-bromosuccinimide (NBS) and pyridine (Py) are used for preparing the leaching solution. It is worthy to mention that the JNMs exhibited lower affinity toward gold in aqua regia system than that in NBS/Py (See Table 10 Practical application of gold recovery from e-waste treated with aqua regia. 150 CPUs were soaked in 500 mL of aqua regia for 24 h. The leaching solution was obtained by filtration and diluted with water to a volume of 1000 mL. After then, the resulted solution was adjusted pH to 2 with NaOH, then 250 mg of JNM-100-AO added and stirred for 1 hours, and finally filter to obtain a gray brown powder JNM-100-AO-Au. The resulted powder was soaking in 100 mL 1M thiourea solution (pH=2, adjusted with HCl) and stirred 5 hours to give yellow solution and JNM-100-AO. After filtration, the JNM-100-AO was added again into leaching solution to collect residual gold, then desorbed with thiourea solution. Such processes were repeated three times. The resulted yellow solution was reduced with Na2S2O5 to give a black powder of gold. The powder was washed three times with 50 mL water and then air-dried. Then, 10 mg of borax was mixed into the powder as a stabilizer and the mixture was sintered at ultra-high temperature until the black powder disappeared to obtain a molten golden yellow solid.
The obtained gold was weighed, and 1 mg of gold was scraped and dissolved in 10 mL of aqua regia, and the purity of gold was determined by ICP-MS.

Gold recovery performance comparison.
Gold recovery performance was evaluated by gold adsorption efficiency, where Ae = (C0−C)/C0 (C0 represents the original gold concentration before adsorption, C represents the remain gold concentration in leaching solution after adsorption). As shown in Supplementary Table10, gold removal efficiency is estimated to be 80% after three adsorption-desorption cycles, which can be improved by further recycles. It is worthy to mention that the Ae is much higher for NBS/Pyridine system than that for aqua regia. Since the JNMs will be positive charged under acidic condition, the high concentration of counterions such as Cl − and NO3 − would be competitive with [AuCl4] − , leading to the lower gold removal efficiency.